This invention relates to improving the beam pattern of an ultrasonic transducer array in the direction perpendicular to the array length.
The radiation pattern from an aperture can be described by diffraction theory. If the pattern is measured in the far field of the aperture, it is the Fourier transform of the aperture function. Thus, for a rectangular aperture the pattern has side lobes at -13 dB (one way). The present invention demonstrates a technique to improve the beam pattern by reducing the energy in the side lobes, and this is achieved entirely within the transducer.
In linear phased array, real time imaging systems, the beam pattern in the image plane and along the array (X-axis) is controlled primarily by the system electronics. The beam pattern in the perpendicular plane (Y-axis) cannot be altered by the system electronics, and is determined solely by the array architecture. Conventional arrays, such as those with long, narrow rectangular elements, have Y-axis beam profiles which exhibit substantial side lobe levels.
Concurrently filed application Ser. No. 349,143, "Ultrasonic Transducer Shading", L. S. Smith and A. F. Brisken, which is assigned to the same assignee, discloses and claims several techniques for shading single element transducers and arrays such that the intensity of emitted radiation is higher at the center of the transducer and lower at the edges and which realize a reduction in side lobes. These include changing the piezoelectric conversion efficiency or polarization as a function of position, having different element lengths, selectively poling the piezoelectric material to yield poled and unpoled regions, and control of electrode geometry. Both X-axis and Y-axis shading are described; the shading function is, for example, the raised cosine or Hamming, and there are many others. Phased arrays may be shaded by the first three techniques. One configuration not suitable for phased arrays is a large slab rectangular element with independent shading of the Y-axis because one electrode covers the whole length and the other electrode covers part of the length.